Wheel Size

[Dinan55]

I don't understand this...

I was looking around on bmwusa.com and I looked at the wheel & tire measurments for the Z4, they read:

18 x 8.0 front/18 x 8.5 rear Ellipsoid cast-alloy wheels (Styling 107) and 225/40R18 front/255/35R-18 rear run-flat tires

And then I looked at the M3 measurements:

M-style alloy wheels with Satin Chrome finish and safe-stop rim design, 18 x 8.0 front, 18 x 9.0 rear, with asymmetric hump; 225/45R-18 front and 255/40R-18 rear performance tires


My question is this, how can the width of the Z4 rear wheels be 0.5" less than the M3 rear wheels, and yet have the same tire width? Also, if you get the M3 with the optional 19" wheels, the rear wheel width is 9.5 inches, and the tires still measure 255/35-19. Either there's something I don't understand, or that site has errors on it. Their new site already has many errors and I'm pretty sure this could be another one.

 

[mikev]

thats strage but its always bugged me that the wheel width is inches and the tires in mm why can't it just be one straight way!!!

after all thats how nasa's second mars lander screwed up!!

 

[frolf]

w/ mm measurements of tires what you do is "get close" i'm sure i could squeeze 235's on my wheels, but the surface of the tire would be super round, and the center would wear really really fast.

 

[Dinan55]

Well isn't that what would happen with the M3? Why do they do this? Is the tread wider??

 

[frolf]

naaa, those tires sizes are fine.